For example in this episode’s poem:
天（そら）に響めく（どよめく） まやかしの
捉う心（とらうこころ）にはしる旋律（せんりつ）
For me it’s interesting that the kanji used for そら (sky/heaven) is not the usual 空 but 天 which is never read そら, usually read てん, but it conveys the meaning of heaven more than 空.
I’ve seen Bleach doing it several times in the poems
I didn’t know about the kanji restriction but that makes sense.
As far as readings, yeah, you’ll definitely see some wacky ones. There’s also a lot names that use ateji and spell things phonetically with kanji like 真理亜 (Maria).